This invention relates to a microphone comprising a housing formed by a cap sealed to a substrate and a MEMS (microelectromechanical systems) die mounted on the substrate. It also relates to a method of manufacturing such a microphone.
This type of microphone, known as a MEMS microphone, can be divided into top-port microphones (with the acoustic inlet port on the top side of the housing and the contacts on the bottom side) and bottom-port microphones (with the acoustic inlet port and contacts on the bottom side).
A cross-section through a bottom-port microphone is shown in FIG. 1. A MEMS die 1 is mounted on a laminate base 2 along with an application specific integrated circuit (ASIC) 3. An acoustic inlet port 4 allows sound pressure waves to move a membrane 5, which forms part of the MEMS die 1. In response to the motion of the membrane 5, its capacitance varies, and this variation in capacitance is detected and processed by ASIC 3. An output signal from ASIC 3 is made available at contacts on the laminate base 2. The volume trapped between the membrane 5 and the cap 6 is relatively large and does not affect the compliance of the membrane 5 significantly. The microphone is therefore quite sensitive and exhibits a high signal-to-noise ratio (SNR). In this design of microphone, the contacts and acoustic inlet port are both provided on the laminate base 2. This can be quite restrictive in some applications, for example if it is desired to have the acoustic inlet port on the opposite side from the contacts.
FIG. 2 shows a cross-section through a top-port design of microphone, in which the acoustic inlet port 4 is provided in the cap 6 rather than in the laminate base 2, thereby overcoming this restriction. This type of device suffers, however, from some significant problems. Specifically, it has a lower sensitivity and SNR due to the small volume behind the membrane (i.e. the volume trapped between the membrane 5, MEMS die side walls and the laminate base 2). This small volume significantly affects the compliance of the membrane 5.
Furthermore, the reliability of the device is poor relative to the bottom-port design. This is because the acoustic inlet port 4 (which has a rather large diameter in the region of 500 μm) exposes the microphone components to the environment. The components are very sensitive to moisture, especially the MEMS die 1, which has circuit impedances in the TΩ range. The reliability can be improved somewhat by application of a hydrophobic varnish to seal ultra-high impedance areas from moisture, but this would have to be done after assembly and wirebonding. As an example, the SPU0410HR5H PB top-port microphone has been found to comply with the Moisture Sensitivity Level Assessment MSLA2a reliability standard, whereas the equivalent bottom-port microphone complies with MSLA1, indicating an improved resilience to moisture. Furthermore, ingress of moisture is not the only concern; dust or other particles entering the acoustic inlet port 4 can also significantly degrade the microphone performance.
Another top-port design is shown in FIG. 3. This is identical to the design of FIG. 2, apart from a channel 7 formed in the laminate base 2. The channel 7 forms an extension to the chamber behind the membrane 5, thereby increasing the volume behind it. This has the effect of enhancing the sensitivity and SNR. However, it has no effect on the poor reliability and it is expensive to manufacture the channel 7.